The present invention relates to a method and apparatus for assembling print heads for ink jet printers and, more particularly, to such an apparatus and method using heat and pressurized fluid to adhere a photopolymer used to bond and space the substrate and orifice plate.
For high print quality, strict tolerances are applicable to the relative spacing and orientation of the substrate and orifice plate of the print head of a thermal ink jet printer. Accordingly, a major objective of the present invention is to provide for more precise control of spacing and parallelism in the manufacture of such print heads.
Thermal ink jet printers comprise one of two major types of drop-on-demand ink jet printers. Drop-on-demand ink jet printers are contrasted with continuous stream ink jet printers which use electrodes to direct ink toward or divert ink from a recording medium, such as paper. Drop-on-demand ink jet printers tend to be simpler than continuous stream printers since the former do not require electrode assemblies to control ink trajectories or recirculation systems to collect and recycle ink diverted from the recording medium. The other major type of drop-on-demand ink jet printers eject ink in response to a control signal by mechanically applying pressure to ink to eject it through one or more orifices.
Thermal ink jet printers, also known as bubble jet printers, have emerged as versatile sources of high-quality printing. Thermal ink jet printers use heat to vaporize some component of the ink to form a vapor bubble. The volumetric expansion of the vapor bubble forces adjacent liquid ink to be expelled through an orifice and propelled toward a recording medium.
Generally, a print head for a thermal ink jet printer includes a substrate and an orifice plate spaced by material which is patterned to define channels through which ink can flow. The substrate is fabricated according to conventional semiconductor processing techniques and includes electrical current paths with resistors in series with conductors. The orifice plate is mounted parallel to the substrate, with orifices generally aligned over respective resistors. When, in response to a control signal, current is caused to flow through one of the resistors on the substrate, sufficient heat is generated to vaporize a component of the nearby ink so as to create a bubble within a respective channel. As ink is expelled through the orifice, the bubble collapses and the print head is soon ready for another cycle.
The relatively quiet operation perceived by a user of such a printer belies a violent and complex flow of events on the scale of the components of the print head. The dimensions of each ink channel, and thus the spacing between the substrate and the orifice plate, must fall within strict tolerances to achieve a precise and predictable droplet trajectory and volume despite the rapid growth and collapse of the vapor bubbles and the turbulence of the ink liquid and vapor.
Tolerances are even more critical in the usual case in which the print head has multiple printing elements and orifices. To create images at reasonable rates, most drop-on-demand ink jet printers use at least nine printing elements. Multi-color ink jet printers can use three or four sets of printing elements for single-pass color printing. For accurate registration of colors and high quality printing, the dimensions of the channels corresponding to each orifice and the alignment of the components must be held to fight tolerances. Failure to hold the orifice plate parallel with respect to the substrate, results in an increase in manufacturing costs as quality control rejects unsatisfactory printers and components.
Addressing the problem of attaching the substrate and orifice plate with these toleranoes in mind, it has been found that most procedures using a separate adhesive are unsatisfactory, at least as to yield. The range of suitable adhesives is limited by the structure and operating environment of a thermal ink jet head. The adhesive must be able to tolerate the thermal, mechanical, and chemical stresses of such an environment. In particular, the adhesive must be compatible with the ink which can be corrosive. Also, the adhesive must be light weight to minimize print head inertia.
One class of adhesives well suited to such an environment are UV-curable polymers. Unfortunately, since the orifice plate is usually opaque, it can be difficult to cure the adhesive once it is in position. Generally, the orifice plate must be positioned prior to the application of adhesive, although screen printing of an adhesive on the substrate or orifice has been attempted without much success. Positioning the adhesive is a delicate task, requiring a syringe or other precise applicator.
In addition to bonding the substrate and orifice, the adhesive is required to complete a seal for the fluid channels. Adhesive applied irregularly to the channel barriers can create a plug in one of the channels, interfering with ink flow or causing ink to leak or, in a color printer, mix. Where the substrate and orifice plate are positioned using a vacuum hold down, a small air gap could cause adhesive to be drawn toward the vacuum source the result being complete or partial blockage of an ink channel.
A "thermal bonding" process has been developed at Hewlett-Packard Company. In a thermal bonding process, heat and pressure are used to fuse one material to a different material. The basic idea is to use the spacer material, which forms the side-walls for the ink channels, as the adhesive. In this approach, a thick film polymer, such as Dupont Vacrel, is patterned over the thin film layers on the substrate and photolithographically processed so as to define the channel barrier structures and a spacer for the substrate and the orifice plate.
Prior to the thermal bonding process, the thick film polymer is partially cured using ultraviolet light exposure of 5-10 Joules/cm.sup.2. The substrate and orifice plate are then aligned and held together temporarily by a small dollop of adhesive, or vacuum or other technique. The patterned substrate and the orifice plate are then clamped between two hard-tooled nominally parallel, surfaces. The substrate and orifice plate are heated so that the polymer barrier becomes "tacky". As the hard-tooled surfaces apply pressure, the softened polymer spacer bonds to the orifice plate. A final high temperature cure cycle further strengthens the bond and completes polymerization.
This "thermal bonding" process works well when the surfaces applying the pressure are parallel and the print head components are of uniform thickness. However, the required tolerances for parallelism and uniform thickness are not easily achieved. These tolerance requirements can be relaxed by inserting a mechanical buffer layer between one of the pressure-applying surfaces and the assembly to be thermallY bonded.
For example, the pressure-applying tool can include a stationary base and a movable upper tool with nominally parallel opposing faces. The movable upper tool can incorporate a heating element for rendering the spacer tacky. The mechanical buffer is a layer of paraffin, or other substance with a low melting point, enclosed between two layers of aluminum foil. This paraffin sandwich can be placed over the components.
The movable upper tool is then lowered to contact the paraffin sandwich Once the paraffin melts, pressure is applied to the components through the paraffin sandwich which conforms to the orifice plate so that pressure is applied uniformly. Thus the paraffin sandwich corrects for small deviations in parallelism.
While the paraffin-sandwich approach appears to work, it has a number of disadvantages. Despite the improved tolerances provided by the paraffin, a high degree of parallelism is still required between the upper tool and the base. However, to maintain adequate parallelism over the range of applied temperatures and pressures requires a high degree of fixture complexity. This complexity, then, impacts cost and reliability of the manufacturing process. Manufacturing costs are further aggravated by the use of an expensive, non-standard, consumable, the paraffin sandwich. Also, this approach puts the print head at risk of contamination if the paraffin leaks.
In addition, the paraffin-sandwich approach lacks flexibility. Process parameters including times temperatures and pressures are sensitive to the area of the components being joined. This makes it difficult to change print head types to be assembled at a given assembly station. It is, a fortiori, impractical to batch process, i.e., join the components of each of many assemblies at one time.
Accordingly, prior to the present invention, there has been a need for an improved apparatus and method for joining print head components. In particular, there is a need for such an apparatus and method which provides for high yields and yet is simple, economical, flexible in application, and mechanically tolerant.